1. Field of the Invention
The present invention relates to an oil seal, and in particular to an oil seal of a two liquid type in which liquid is present on both sides of the oil seal.
2. Description of Related Art
Heretofore, an oil seal fitted on a rotary shaft has a rubber lip which is urged so as to be radially pressed in order to prevent lubrication oil or the like from leaking around the rotary shaft. There have been known several kinds of oil seals including a one liquid type one having a single lip for preventing lubrication around a rotary shaft from leaking from one side to the other side of the oil seal, and a two liquid type one having lips provided on both side of the oil seal, for preventing lubrication oils on the respective sides from leaking from one side to the other side or vise versa. The two liquid type oil seal is provided, for example, between the outer periphery of a crank shaft of an engine and the inner periphery of a flywheel in order to seal against engine oil fed onto the crank shaft and lubrication oil fed onto a clutch disc on the flywheel side or a transmission.
In the above-mentioned two liquid type oil seal, each of both lips is formed on a surface thereof making contact with the rotary shaft with a spiral groove for forcing the lubrication oil having entered between both lips toward the outside of the oil seal. The lubrication oil trapped between both lips is driven into the grooves when the rotary shaft is rotated, and is then spirally forced out by the rotation. During the operation in a normal condition, negative pressure, which is induced in a space defined by both lips and the rotary shaft as the lubrication oil is driven out, is made up for with air sucked thereinto through gaps between the lips and the rotary shaft.
By the way, as to the material of the two liquid type oil seal, there has been known a fluororubber, silicone rubber, nitric rubber or the like. However, since the oil seal is used making contact with a rotary member, it is likely to wear being susceptive to a use condition including a rotational speed and a temperature, and accordingly, the development of a material which is excellent in wear-resistance has been desired. Thus, these years, oil seals made of polytetrafuluoroethylene (PTFE) have been used prosperously. This material is nonadhesive and less frictional, and is excellent in wear-resistance and heat-resistance in comparison with conventional rubber so as to be frequently used for oil seals in engines which are used at a high temperature. An oil seal made of PTFE has lips which have a high stiffness and are hardly deformed, and accordingly, even though negative pressure is induced, air can be soon introduced through gaps between the lips and a rotary shaft so as to satisfactorily make up for the negative pressure.
However, under such a condition that lubrication oil is always fed to the rotary shaft and the oil seal due to a state of excessive lubrication, the lips of the oil seals are always soaked in the lubrication oil. In this case, the oil seal continuously forces the lubrication oil toward the outside thereof, and accordingly, even with an oil seal made of PTFE, it would be caused that negative pressure is inevitably induced in a space defined by the lips and the rotary shaft.
In the above-mentioned condition, the bearing of the oil seal becomes excessively high due to the negative pressure, and accordingly, abnormal heat would be caused at the sliding surfaces of the lips. Due to a temperature rise, lubrication oil at the sliding surfaces of the lips is deteriorated so that the spiral grooves are filled therein with deterioration product, and accordingly, the oil seal fails to normally force the lubrication toward the outside thereof, resulting in deterioration of sealing. Further, due to an increase in the bearing of the oil seal, abnormal abrasion is caused so as to determine the durability of the oil seal.